How to create piles of snow on a mesh?
There is no doubt that the best result can be generated by using Particle or Physics system, which is a bit complicated to talk about in a few words. A quick-and-dirty way is shown as below:


Switch Transformation Orientation to Normal, try to find and select a single vertex which Z axis towards up (or nearly up). Press ShiftG1 to select nearly all possible upward vertices by tweaking a reasonable Threshold value, as shown in Figure 1 and 2.
ShiftD to duplicate the selection, then P1 to separate it as a new mesh. Now you can freely tweak it with a few modifiers to make a nice pile-looking snow cover, particularly with the Displace modifier. As shown in Figure 3.